Adhesive, selection

Obviously some adhesive mechanical property data must be specified together with a detailed elaboration of the test procedures used for obtaining them, preferably over a range of conditions. 

Procedures for the determination of important properties are essential for the selection of an appropriate product and for joint design and analysis. We have seen that adhesive polymers are sensitive to temperature and moisture, and to the rate at which stress is applied. Their bondline behaviour in joints depends greatly upon the system stiffness, and upon more subtle variations such as bondline thickness. It is therefore important that test methods are relevant to the real application, the fabrication conditions, the actual materials to be joined, and so on. Many standard test methods exist, for both strength and durability assessment, and these were discussed in Chapter 4. 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

The working characteristics of the adhesive relevant to the application conditions must be determined. For instance viscosity is often temperature, shear-rate and time-dependent, and this will influence the choice of dispensing equipment, the method of application, the usable life and the open time. The viscosity should therefore be regulated bearing in mind the adherend rugosity and surface pretreatment, the method and location of application, and the cure temperature and duration of application. A thixotropic material may be required for application to vertical or soffit surfaces. Generally, relatively thick bondlines are encountered so that the adhesive should be able to cure in thick and/or uneven layers. It should also be remembered that for about every 8 C change in 

It is important that the adhesive manufacturer should be assessed for the quality and consistency of his products. Quality control test data should be available for each batch of adhesive, which requires the prior definition of the necessary characteristics and the means of assessing them. Adhesive materials should be packed in suitable containers ready for mixing, and each container should be durably and legibly marked with appropriate information. An instruction sheet should also accompany any delivery of adhesive, detailing information which includes product chemical type, storage conditions, mixing and application instructions, advice on compatible primers, curing profiles, safety instructions, and so on (e.g. see Appendix), 

The first two are usually of interest to the adhesive supplier, while the third is of particular interest to the user of the adhesive. 

Tests to monitor the quality of production materials (QC tests) have relied on checking such properties as viscosity, percent solids, sag, pot life, and cure rate, in addition to measuring mechanical properties. The same tests have traditionally been used for receiving inspection procedures by the adhesive user however, there is now interest, particularly in the aircraft industry, to augment these by the use of physicochemical methods of analysis. These newer methods of control are expected to provide information of a more fundamental nature, useful in characterizing adhesives and diagnosing the causes of performance anomalies. 

One problem, always present in the use of structural adhesives, is how to determine the reliability of the bonded structure. This may be at the time of manufacture, to ensure that a good joint has been produced, or during service, to monitor behavior under operating conditions. It is often impractical, or impossible, to use the types of test methods employed to determine mechanical properties and interest has centered on nondestructive test methods.  

Assuming that the decision to use a structural adhesive to solve a joining problem has been taken, the question then arises, which adhesive should be used Many factors have to be taken into account in the selection of the appropriate adhesive for any particular application. It is difficult to generalize, since a property that is considered an advantage in one application may be a limitation in another. 

In most cases, the ability of the adhesive to support the design loads under service conditions for the planned life of the structure is considered first. Thus the mechanical properties, durability, and environmental resistance of the bonded structure are of obvious importance. But of equal concern are the nature of the adherends, the application technique, the cure conditions, the handling requirements, and, perhaps, the cost. A list of some of the factors considered in the selection process, together with the properties of the major generic classes of adhesives, are given in Table VI. It is important to remember that, even within a given class of adhesives, the performance may vary considerably, so that it is essential to consider the properties of each individual adhesive. It is apparent from this brief discussion that adhesive selection is far from simple. A computer-based selection program has become available recently.  

The selection procedure in Section 5.2 is arranged to guide the user towards the most suitable adhesives and their manufacturers. For the best and quickest results study the background information given in the relevant sections of this handbook. 

The main value of this selection procedure is in mechanical and structural engineering - the requirements of mass production are given particular emphasis. Experience has shown that, in these areas, the procedure will almost always identify a satisfactory adhesive type. 

Elsewhere, the proposals will contain a sensible solution and, as a whole, better the likely performance of most experts - since expertise is often narrow and influenced by commercial considerations. Throughout, every effort has been made to ensure that the information is correct - in the widest sense - and that the procedure does not favour the products of any one manufacturer. 

For the purposes of this review and the process of selection, it is convenient to classify adhesives into the following broad family groups. 

Hot melt Phenolic Resorcinolic Phenolic/resorcinolic Modified phenolic 

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Adhesive Selection Chart, McGraw-HiU Book Co., Inc., New York, 1971. 

The aerodynamic wheel cover of claim 1 wherein said adhesive means comprises an adhesive selected from the group consisting of velcro, adhesive tape, or an adhesive resin. 

Steinberg, M.S. (1964). The problem of adhesive selectivity in cellular interactions. In Cellular Membranes in Development, (Locke, M., Ed.), pp. 321-366. Academic Press, New York. 

The overall system quality is often very much dependent on the quality of the adhesive selected. 

End users of adhesives and medical devices expect the device to withstand the rigors of sterilization, exposure to fluids, and occasional abuse. Because of the criticality of the product, the medical device manufacturer is generally more motivated than most to pay attention to adhesive selection criteria and the requirements of good bonding practice. 

Morris, C. E. M., Strong, Durable Adhesion Bonding Some Aspects of Surface Preparation, Joint Design, and Adhesive Selection, Materials Forum, vol. 17, 1993, pp. 211-218. 

Kim, W. H., Jun, S. H., Kibbey, M. C., Thompson, E. W. and Kleinman, H. K. (1994). Expression of beta 1 integrin in laminin-adhesion-selected human colon cancer cell lines of varying tumorigenicity. Invasion Metastasis 14, 147-155. 

Maddox JE, Serhan CN (1996) Lipoxin A4 and B4 are potent stimuli for human monocyte migration and adhesion selective inactivation by dehydrogenation and reduction. J Exp Med 183 137-146... 

In the following, we therefore attempt to provide both fields of application with suitable information, since successful and precise bonding is the aim of all users. In addition, adhesive manufacturers have a comprehensive range of information material on adhesive selection and adhesive processing for the products they offer. 

The information is limited to the most important materials, metals, plastics, (thermoplastics, thermoset materials, foams), ceramics, glass and their possible combinations. For papers, cardboards, wood, rubber polymers, usually physically setting systems (solvent-based, dispersion, hot-melt adhesives) are utilized. In these cases, the adhesive selection with regard to the manufacturing conditions and demands is of less problematic nature. 

The different possibilities of surface pretreatment are left unconsidered in the systematics of adhesive selection. Except for very special conditions regarding climate and humidity in case of long-term effects, which require expensive chemical and electrochemical treatment, it is assumed that the process combination ... 

Note In the case of metal bonded joints, the resistance to moisture and climate can decisively be influenced by the kind of surface pretreatment apart from the adhesive selection, especially in the areas adjacent to the adhesive surface (e.g., primer, sealing of gluelines). 

With regard to the economic requirements of adhesive selection, it is a general rule... 

Easily deformable, especially Cu sheets, thus deformation characteristics of adhesive layers important for adhesive selection. 

Independent of the chemical structure, industrially available adhesives are characterized by the formation of strong adhesive bonds on the respectively pretreated surfaces of the materials described. This results in the criteria for adhesive selection described in Chapter 8. 

The following summary should be observed in adhesive selection ... 

To avoid inner tensions through thermal stress, it is recommended to use only adhesives that cure at room temperature. The adhesive selection is limited by the fact that many applications require an invisible glueline. In such cases, adhesives with fillers are excluded, cyanoacrylates and in particular radiationcuring products (Section 9.3.3) are the suitable choice. If the visual appearance of the bonded joint is not important, two-component reactive adhesives based on expoxides, polyurethanes, methacrylates, contact adhesives and, if required, adhesive tapes are recommended. 

Doyle, Daryl J. (1990). Criteria for Proper Adhesive Selection From Application to Viscosity. Society of Manufacturing Engineers Technical Paper AD90-450, Adhesives 90, October 1-4, 1990, Schaumburg, Illinois. 

Table 2.9 gives a Kst of various adhesives and typical applications in bonding of plastics. This table is not complete, but it does give a general idea of what types of adhesives are used and where. It should be remembered, however, that thousands and thousands of variations of standard adhesives are available off the shelf. The computer may shape up as an excellent selection aid for adhesives. Selection is made according to the combination of properties desired, tack time, strength, method of appKcation, and economics (performance/cost ratio). 

DM 3030, Silver/glass die attach paste. Thermally Conductive Adhesives Selection Guide. Diemat, a Namics Co, www.diemat.com 2003. 

Estes R. A practical approach to die attach adhesive selection. Hybrid Circuit Technol. Jun. 1991. 

Besides an understanding of the basic failure modes and mechanisms, important in the initial selection of adhesives, qualification and strict adherence to materials and processes specifications and work instructions are an integral part of the rehability process. The process begins with the design guidelines and requirements for a specific application. Qualification criteria and test specifications must then be generated and the adhesives selected must be qualified. In addition, materials specifications or... 

VINCENTZ VERLAG Schiffgraben 43, D-30175 Hannover Germany, phone -1-49 (511) 9910-272, fax -t-49 (511) 9910-279. Adhesion Congress Tracking Adhesion— Selecting the Right Binders. 

Advances in the use of TP and TS plastic adhesives have made possible the adhesive bonding of RP structural and nonstructural parts in appliances, automobile, aircraft, medical devices, and so on. Adhesives with strengths higher than some metals are used (epoxy, etc.). The wealth of adhesive technologies that are available could make adhesive selection a task if one does use the proper approach such as determining specifically what performance requirements are needed (as with any selection procedure). The best adhesive for an application will depend on processing considerations and meeting the performance requirements. Tables 5.23 and 5.24 provide information on types and use of adhesives. 

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